1. Field of the Invention
The present invention relates to a saline and sewage water reclamation system and process using an extremely efficient vapor compression/vacuum distillation cycle. Though the system and process has its greatest use with saltwater or contaminated water, it also can be used to reclaim other fluids or to remove toxic wastes from fluids.
2. State of the Art
Distillation is a common method known to remove unwanted substances from a contaminated water supply or to remove salt from brine. The process occurs by a selective phase change between the differing vapor pressures of the contaminants and the water vapor. Phase change by evaporating water is the process by which rainwater has been recycled continuously since water first appeared on earth. The earth""s water bodies are open systems. Consequently, the balance of differing vapor pressures between the water body and the atmosphere and the heat flux from solar radiation acting on the water body affects the amount of evaporation.
Distillation is slow at atmospheric pressures unless the heat flux is raised to the boiling point of water, 212xc2x0 F. (100xc2x0 C.) at sea level. (Metric conversions are approximations.) Therefore, to distill water at atmospheric pressures, heat energy must raise the temperature from ambient to 212xc2x0 F. (100xc2x0 C.). At that temperature, water boils and vaporizes, changing from liquid to gas. Once the water vaporizes, a cold source must be present to condense liquid water from the water vapor. One must use additional energy to remove heat from a cold trap and create a continuous cold source to condense the fluid.
Boiling contaminated liquid at atmospheric pressures usually has not been economically viable. For desalination or waste management, the temperature change is from about 70xc2x0 F. to 212xc2x0 F. (21xc2x0 C. to 100xc2x0 C.), a 142xc2x0 F. (61xc2x0 C.) temperature difference (xcex94T). In colder climates, the temperature difference often is greater. The necessary energy required to heat water to boiling and to maintain condensers cool enough to condense the vapor makes traditional water distillation systems prohibitively expensive to operate without using costly, xe2x80x9cmulti-effectxe2x80x9d boiling chambers. They have found their niche in specialized applications. For example, in desert regions near an ocean or soda lake, for ships and in space applications, one may trade off the high energy cost for the need for potable water. One also may accept the high energy costs where the contaminants are so toxic that they must be removed from the water.
Not only is higher temperature distillation expensive, it can cause an additional problem. When processing contaminated liquids that contain minerals or organic molecules, higher temperature can cause chemical reactions between the molecules. Some reactions can form high molecular weight molecules that can obstruct boiler walls and make cleaning difficult. High temperatures also break down the walls of organic cells within the contaminates, which can release toxic materials into the liquid. High temperature boiling can cause some lower molecular-weight contaminates to vaporize and migrate with the water vapor toward the condenser.
Reverse osmosis (R.O.) systems also are common for saline water reclamation. They also are costly and are not used for large applications.
Despite problems with ambient-pressure distillation and R.O., desalination capicity in the United States has increased. According to the Office of Technology Assessment, in 1955, for example, the United States had almost no capacity, and less than 30 million gallons per day (Mgal/d) (113.5 million liters per day) could be produced in 1970. By 1985, capacity exceeded 200 Mgal/d (757 million liters per day). Still, that amount is quite small compared to the annual water use in the United States. For example, the United States Geological Service reports that overall fresh water withdrawals in the United States in 1995 was 341,000 Mgal/d (1.29xc3x971012 liters per day).
Conventional distillation systems use conventional boilers. Boilers are an advanced art whose efficiencies have been studied and documented. See, e.g., McAdams, W. H., Heat Transmission 2d Ed., McGraw-Hill 1942, pp.133-137.
Boiler and Condenser Phase Change Processes: The energy required to produce a liquid-to-gas phase change is defined by the heat of vaporization equation given by:
Q=wxcex94hv.xe2x80x83xe2x80x83(1)
Where:
Q=Heat energy; (BTU)xe2x80x83xe2x80x83(1a)
w=Weight of fluid to be vaporized; (lbs)xe2x80x83xe2x80x83(1b)
xcex94hv=Heat of Vaporization of the fluid (BTU/lb).xe2x80x83xe2x80x83(1c)
During a continuous feed flow process, the required energy per unit time is simply the time derivative of equation (1) and defined by:
{dot over (Q)}={dot over (w)}xcex94hvxe2x80x83xe2x80x83(2)
where, {dot over (Q)}=Heat energy flow; (BTU/hr)xe2x80x83xe2x80x83(2a)
{dot over (w)}=xe2x80x9cMassxe2x80x9d (weight) flow of fluid vaporized (lbs/hr).xe2x80x83xe2x80x83(2b)
The heat transfer rate flowing between the boiler and condenser is by a combination of both convective and conductive processes and is given by Newton""s Law of Cooling defined by:
{dot over (Q)}=UAxcex94T,xe2x80x83xe2x80x83(3)
where, U=Overall heat transfer coefficient; (BTU/(ft2hr xc2x0 F.))xe2x80x83xe2x80x83(3a)
A=Overall boiler and condenser area; (ft2)xe2x80x83xe2x80x83(3b)
xcex94T=TCxe2x88x92TB=Temperature difference between boiler and condenser (xc2x0 F.).xe2x80x83xe2x80x83(3c)
One computes the overall heat transfer coefficient by the standard parallel addition of local, individual heat transfers, which yields:                               1          U                =                              1                          h              C                                +                      1                          h              B                                +                                    1                              (                                                      k                    wall                                                        t                    wall                                                  )                                      .                                              (        4        )            
See, e.g., McAdams, W. H., Heat Transmission, 2d Ed., McGraw-Hill 1942, pp. 133-137. This expression has the following definitions:
hC=Condenser local heat transfer coefficient; (BTU/(ft2hr xc2x0 F.))xe2x80x83xe2x80x83(4a)
hB=Boiler local heat transfer coefficient; (BTU/(ft2hr xc2x0 F.))xe2x80x83xe2x80x83(4b)
kwall,kfluid=Thermal conductivities of wall and fluid; (BTU/(ft hr xc2x0 F.))xe2x80x83xe2x80x83(4c)
twall,tfluid=Thickness of the common wall (ft).xe2x80x83xe2x80x83(4d)
The mass flow, {dot over (w)}, in pounds per hour of purified fluid from the distillation unit can be computed by combining equations (2) and (3) yielding:                               w          .                =                                            UA              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              T                                      Δ              ⁢                              xe2x80x83                            ⁢                              h                v                                              .                                    (        5        )            
In units of gallons per day, the mass flow, {dot over (W)}G equals:                                           w            .                    G                =                              C            G                    ⁢                                                    UA                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                T                                            Δ                ⁢                                  xe2x80x83                                ⁢                                  h                  v                                                      .                                              (        6        )            xe2x80x83where, {dot over (W)}G=Mass flow rate; (Gal/day)xe2x80x83xe2x80x83(6a)
CG=Constant conversion to gal/day=(24/8.3454).xe2x80x83xe2x80x83(6b)
Quantities in each preceding equation are temperature and pressure dependent. Consequently, the optimum thermodynamic cycle for contaminated water or any other fluid depends on the fluid and contaminants. Most fluids have known properties, however. Accordingly, one can account for the particular fluid. Further, a computer microprocessor feedback and control system can adjust for any specified requirements.
Heat Transfer Performance: Equations (5) and (6) show that a linear increase in the overall heat transfer coefficient U increases the system output flow rate linearly. Increasing the temperature difference requires added energy consumption. The ambient input temperature, which is not controlled, determines the working temperature. Therefore, maximizing the heat transfer coefficient without increasing the working temperature T or the temperature difference xcex94T is advantageous.
Thin boiler wall thickness: It is important to utilize a very thin boiler/condenser wall surface thickness twall, with metals that have high heat conductivity kwall. Typically, the wall thickness is between 0.010 inches to 0.015 inches (0.25 mm-0.38 mm). The heat conductivity for steel, a typical boiler wall surface, is about 25 BTU/(ft hr xc2x0 F.) (0.43 watt/cm-xc2x0 C.), which yields a boiler wall conductivity heat transfer rate of between 20,000 and 30,000 BTU/(ft2hr xc2x0 F.).
Thin Film Boiling: Minimizing the contaminated water fluid film thickness against the boiler surface improves heat transfer to fluid in a boiler. Conventional boilers do not create a uniform thin fluid film against the boiler surface. Consequently, they must rely on high temperature gradients to conduct heat through the fluid film. Thin film boiling normally operates at lower boiling temperature. In the prior art, the thin film of liquid is deposited along the boiler wall in two different ways, wiping or spraying.
By rotating the entire assembly, centripetal xe2x80x9cgxe2x80x9d loads cause the fluid to form a thin uniform film against the boiler surface. In the present invention, injectors adjust the liquid film thickness in accordance with the boiler rotation rate. Computer feedback logic could maximize the purified output flow rate to the prescribed energy consumption. Applicants recognize that rotation causes centripetal loads. In common parlance, many refer to this load as centrifugal force. Though no centrifugal force exists to act on the liquid, the application still uses the term xe2x80x9ccentrifugalxe2x80x9d to denote forces causing liquid to film on the inside on the boiler shells.
Fluid film thickness values are usually maintained between about 0.020 in to 0.010 in (25mm to 0.51 mm). Conventional boilers do not maintain this small of a film thickness. Consequently, their throughput flow rate is limited to operating regimes at high xcex94T temperature differences or large boiler surface areas A because the boiling heat transfer rates are reduced.
High boiler heat transfer Under conditions of relatively low fluid velocities (low Reynolds numbers, Re), convection and phase change processes govern the boiling heat transfer rate. Thin film conditions in the boiler create a condition whereby nucleate boiling can occur at low xcex94T, which produces high heat transfer rates. The heat transfer process with phase change is more complicated than the normal liquid phase-only convection process. In liquid-phase convection, one can describe the methodology by including the fluid effects of viscosity, density, thermal conductivity, expansion coefficient, and specific heat along with the geometry of the system. However, the mathematics for heat transfer with a phase change also includes the surface roughness characteristics, the surface tension, the latent heat of evaporation, the pressure, the density and other properties of the liquid-vapor. The entire process becomes so complex that empirical experimental data and dimensional analysis determines the analytical expressions.
The weight of fluid against a boiler surface is a major cause that allows heat transfer to take place. Therefore, artificially increasing the weight of the fluid by subjecting it to a rotating xe2x80x9cgxe2x80x9d field against a cylindrical surface causes the boiling heat transfer rate to increase.
High condenser heat transfers: In general, the physical processes that occur when pure vapor condenses are complex. The processes involve a coupled transfer of heat and mass in which the latent heat of condensation provides the heat to be transferred, and the vapor and condensate are the transported mass. Condensing heat transfer rates typically are high due to the large latent heat energy contained in the vapor.
When the condensate remains as drops on the condenser surface, the process becomes less efficient. The drops remaining on the surface prevent new vapor from reaching the surface. Therefore, removing drop-wise condensation from the condenser surface increases efficiency. If the condenser surface is cylindrical and faces outward, rotating the condenser at a high g causes the condensate to be thrown off the condenser wall. As the velocity increases, condensate is thrown off the wall more quickly and completely to yield a surface that is ready to receive vapor.
The principal object of the present invention is to disclose and provide an efficient water reclamation system that can distill brine and other contaminated liquids at low cost. By reducing the system""s operating pressure to near the vapor pressure of the contaminated fluid, boiling can occur at ambient temperatures with a small temperature difference (xcex94T), e.g., 6xc2x0 F. (3xc2x0 C.) or less. By having low temperature boiling occur on one side of a thin boiler wall and condensation occur on the other side of the wall, heat transferred to the wall by the condensing fluid can provide energy for boiling on the opposite side of the wall. Further, constructing the boiler/condenser wall as a cylindrical shell, having the boiling surface face toward the axis of rotation of the cylinder and then rotating the cylinder about the axis, heat transfer for boiling improves due to the higher g forces on the fluid. Likewise, because the condenser surface faces outward, as vapor condenses, the drops of condensate are thrown off the condenser surface. That leaves a clean surface to receive vapor, which improves condenser efficiency. Because of the high heat transfer capabilities possible from a rotating boiler/condenser, boiling can occur practically at ambient temperatures. Low temperature boiling minimizes scale and fouling of the boiling wall surface. This allows the boiling heat transfer rate to be maintained at a very high level.
One object of the present invention is to maintain fluid film thickness values between about 0.020 in. to 0.010 in. (0.51 mm-0.25 mm). The thin film under accelerated g rotation improves boiling heat transfer rate to several thousand BTU/(ft2 hr xc2x0 F.).
Another object of the present invention is to disclose and provide a water reclamation system that processes large volumes of fluid in a small system volume. By arranging the cylindrical shells concentrically, the system of the present invention can accomplish that object.
Another object of the present invention is to design the system to work continuously instead of in batch mode. Thus, contaminated fluid enters the system, and clean water or other purified fluid exits the system and are collected continuously. Likewise, the salts or other contaminates also exit the system continuously and are collected separately.
Some or much of the contaminated fluid likely will not vaporize. Thus, the system may not convert 100% of incoming brine, for example, to potable water. As the percentage converted increases, the boiling point rise increases, and the energy requirements of the system increase. This occurs because as the salinity concentration increases a greater temperature increase is required to boil the remaining fluid. Brine from seawater is inexpensive, and returning brine back to the ocean at slightly higher salt concentrations usually is acceptable. Accordingly, an object of the present invention is to allow it to keep the percentage of brine converted well below 100%, use less energy and be very economical. On the other hand, where the system is removing toxic wastes which must be stored or disposed, limiting the volume of the output (i.e., limiting the amount of fluid that remains with the contaminate) probably is desirable. Therefore, these systems may process and vaporize a higher percentage of these contaminated fluids. Accordingly, another object of the present invention is to design a system that can process different types of fluid from brine to highly toxic waste.
Another object of the present invention is to disclose and provide a construction for the present invention in which the various parts can be constructed at relatively low cost. The present invention uses concentric cylindrical or tapered shells. One object of the present invention is to provide low-cost methods for constructing the shells, including providing shells of different diameters.
Another object of the present invention is to use an efficient compressor for raising the temperature and pressure on water vapor slightly and directing the vapor to the condensing surfaces.
Another object of the present invention is to disclose a process that requires lower power than other systems.
Another object of the present invention is to provide water or other fluid reclamation that can be fully automatic, uses no expendable materials during fluid processing and has a long operational life with minimum maintenance.
To accomplish these objects, the present water reclamation system comprises a series of concentric thin shells. The shells mount within a housing that can be maintained under vacuum. Two adjacent shells form a boiling space, and one of those shells and the next adjacent shell form a condensing space. Thus, the boiler and condenser share a common wall. The shells also rotate together within the housing.
One end of each boiling space is open to a compressor, which raises the pressure and adds heat to the water vapor after the water boils. As fluid boils, the compressor transfers the vapor to the condensing spaces, which are open to the downstream side of the compressor. The vapor strikes the condenser wall, which is a wall that is common to the adjacent boiling space. The vapor condenses and transfers heat to the wall or shell. That heat energy boils new, incoming liquid. This minimizes the heat energy that the system uses. In other words, the latent heat of condensation in the condenser is transferred and then used as the latent heat of vaporization in the boiler. The system needs no heat sources other than the energy of vapor compression to complete the cycle flow from vaporization to condensation.
The boiling surfaces of the shells face the axis of rotation. Therefore, any liquid on the boiling surface receives added g forces. By controlling the fluid flow onto the boiling surface, the system also can maintain the fluid as a thin film. Thin film boiling and added g forces increase the heat transfer rate.
Because the condenser surfaces face outward, when vapor strikes the surface and condenses, it immediately is thrown off the surface. This leaves a clean surface on which new vapor condenses. High g forces cause the condensate, now a pure liquid, to collect as a film along the outer shell of the condenser space. Tapering the shells causes the condensate to flow toward the larger diameter end of the shell where it is collected.
As the liquid boils in the boiler, the high g forces maintain the contaminates on the boiler wall. The system""s construction prevents the contaminates from flowing back toward the inlet. High g forces also cause the contaminates to flow along the boiler wall. When they reach the end of the wall, they are thrown outward and collected in a ring. From the ring, the contaminates are pumped or otherwise directed out of the housing. To keep the sludge and pure liquid separate, collection of contaminates and pure liquid can occur at opposite ends of the shells.
The system can deliver ultra pure water or other liquids, and it can be used for toxic waste clean-up applications. There, the system fractionally separates all kinds of liquid contaminates including nitrate polluted wells, cyanide polluted mines, petroleum polluted water tables and even radioactive contaminants contained in water. In industrial applications, textile factories could separate their contaminated dyes and solutions from water, plating industries could separate their metals and chemicals from water used in their plating troughs, and electronic industries could separate the toxic chemicals from solutions used during their manufacturing process. All these applications can be handled with the same processing technology functioning under different but specific operating conditions.
These and other objects are evident from the description of the exemplary embodiment of the invention.